The quality of the fuel used to drive internal combustion engines may be subject to manufacturing tolerances, for example the fuel may contain varying proportions of long-chain hydrocarbons.
It is therefore a well-known procedure to determine the quality of the fuel used and to take this into account in the fuel injection process in order to compensate for the quality variations as far as possible.
At present, variations in fuel quality are detected using a starting-quantity adaptation technique or the uneven-running method, and compensated for by varying the injection periods. In these known methods, the increase in speed when starting the internal combustion engine or the speed variations at start-up are evaluated. If the increase in speed or the speed variations lies/lie outside an allowed preset confidence band the injection period is corrected accordingly. The correction amount is calculated relatively imprecisely, however, so that sometimes the correction proves too great. In particular, given a “poor start”, the injection period is changed so as to make the fuel-air mixture richer, which means that the fuel-air mixture may be too rich after filling up with a very good quality fuel. As a result, the internal combustion engine starts badly or not at all.
Another disadvantage of this method for determining the fuel quality is the relatively low accuracy, because the increase in speed when starting the internal combustion engine does not depend solely on the fuel quality but is also affected by other parameters such as the internal friction of the internal combustion engine.
DE 40 27 947 A1 describes various ways in which one can take into account fuel quality variations. One way is to measure the control deviation in the lambda control circuit after each tank fill of the vehicle driven by the internal combustion engine, and to vary an adaptation value so as to reduce the control deviation to zero. This method has the disadvantage that it only works when the lambda control system is enabled. This is specifically not the case, however, when the internal combustion engine is cold. Other methods are proposed that ensure that, even when the internal combustion engine is cold, the internal combustion engine is still able to run when the fuel composition changes sharply at tank fill-up, for instance when a tank containing fuel is practically emptied and then filled with a fuel containing mainly methanol. Using the tank levels before and after tank fill, an assessment of what sort of fuel compositions can be present is made on the basis of commercial fuel data. The input control values are then modified for running the internal combustion engine using fuels of the possible compositions, and the system assesses which composition the internal combustion engine runs best with. These values are used from then on for controlling the internal combustion engine.
The disadvantage of these known methods for determining the fuel quality, however, is that they are coupled with the combustion process and hence may also be affected by other parameters.